Data transducer head positioners employ both open and closed servo loop technologies in order to move the data transducer head among concentric data storage tracks of a rotating storage disk coated with magnetic storage media during track seeking operations, and to keep the data transducer head aligned with each selected data track during track following operations when data is being read from or written to the data track being followed. The head positioner structure includes an actuator for translating electrical driving currents into motion to move the head back and forth in a general radial direction across the data storage surface, and to maintain the head in each selected track following position. A driving amplifier converts position error signals into the driving currents which flow through the actuator coil. The position error signal may be developed by servo feedback information provided from a dedicated servo surface of the disk, or it may be provided on a sampled and held basis from servo sectors recorded in interleave fashion with data sectors in one or more of the concentric data tracks, or it may be derived from an external transducer, such as a polyphase optical transducer coupled to the actuator structure.
It is known within disk drives to sense temperature as a direct measure of changed environmental conditions and provide an error value based on sensed temperature to correct for such temperature changes. See, for example, U.S. Pat. Nos. 3,753,254 and 4,207,601, which describe the use of temperature sensors, such as thermistors, to provide a mechanism for correcting errors in head position as sensed by an actuator position sensing mechanism due to thermal variations.
U.S. Pat. No. 4,136,365 describes a disk drive employing reference tracks and a timer which causes the head positioner servo loop to seek to and read servo correction values from the reference tracks during interruptions in read and write operations in order to recalibrate a position transducer coupled to the actuator. An inductive position sensor is described as preferred. The timer interval is established as a function of the resultant error value derived from reading the reference tracks: the greater the error, the more frequently will check cycles be carried out. This method provides some indirect measure of the rate of thermal changes within the disk drive.
Commonly assigned U.S. Pat. application Ser. No. 07/192,353, filed on May 10, 1988, now U.S. Pat. No. 5,005,089, describes a fixed disk drive servo system including a mass balanced rotary voice coil actuator, a polyphase optical encoder tightly coupled to the actuator, position correction information sampled from each data surface and held for thermal correction, and a temperature sensor for sensing temperature changes within the disk drive thereby to indicate the need for execution of a recalibration operation as temperature shifts are detected. The disclosure of this prior application is hereby incorporated herein by reference thereto.
The performance of the servo loop positioner is dependent upon temperature. As noted above, the typical head transducer positioner includes an actuator coil, printed circuit traces or wires leading from a driver amplifier to the coil, and a sense resistance in series therewith, across which current flow is sensed. The total resistance seen by the actuator power amplifier driver is the sum of the lead resistance, sense resistor resistance and actuator voice coil resistance. This total resistance increases as the temperature increases.
Without thermal feedback compensation, the servo system must be designed for worst case temperature in order to have proper performance and settling characteristics at the worst case (slowest) condition. Therefore, servo performance is limited to and bounded by the worst case temperature specification. However, most of the time, the servo system is operating at a normal ambient temperature condition, such as 30 to 40 degrees C. The servo design margin necessarily included for worst case performance has not heretofore been reduced for normal and low temperature environments. Also, the operating temperature range specification has been compromised without thermal feedback compensation in order to meet seek error rate requirements.
Thus, a hitherto unsolved need has arisen for a servo system for a disk drive which provides direct thermal feedback thereby enabling performance compensation tailored to the instant ambient temperature.